It has long been known in the field of disposable absorbent articles that it is desirable to construct absorptive devices, such as disposable diapers with fasteners, pull-on diapers, training pants, sanitary napkins, pantiliners, incontinence briefs, and the like, with stretch laminates to improve the ease of motion and maintenance of a sustained fit. Stretch laminates allow the disposable absorbent article to accommodate a range of different sized wearers. A disposable absorbent article may have stretch laminates in a number of its structures including the waist band, leg cuffs, side panels, elasticized topsheets, backsheet, ears, and fastening system.
Various vapor permeable, liquid impermeable polymeric films are known in the art. For example, one method of making a polymeric film vapor permeable, involves mixing a matrix polymer with a quantity (e.g., 10-70% by weight) of an organic or inorganic particulate filler such as, for example, calcium carbonate, and extruding a film from the blend. The matrix polymer may include a polyolefin, such as polyethylene or polypropylene, or various olefin copolymers. The film may be a monolayer film, a multilayer film which contains the polymer/filler matrix as a primary layer along with thin breathable skin layers, or a multilayer film having more than one polymer/filler matrix layer. The film may be stretched. The particulate filler serves as nucleation sites causing the formation of voids in the film. However, films and laminates made from the films generally are not elastic. Even if an elastomer is used in lieu of the polyolefin, the relatively high concentration of particulate filler often degrades the elastic character and tensile properties of the resulting film.
Another common method for making a polymeric vapor permeable film involves aperturing the film during the formation process. For example, needle-punching is a well known process for aperturing films. Apertures may also be formed by subjecting the film to fluid pressure such as a water jet or air jet. While these methods may be used to make elastomeric films breathable, the resulting apertured elastomeric film may exhibit reduced tensile properties. In particular, apertured films often have reduced tensile strengths compared to equivalent non-apertured, monolithic films. In addition, apertured films are, in general, more susceptible to tear and tear propagation while being held at a constant strain when compared to non-apertured films.
Even when breathable elastomers and breathable stretch laminates are formed, these products may not exhibit mechanical characteristics ideally suited for use in disposable absorbent articles. One critical characteristic for stretch laminates for use in disposable absorbent articles such as disposable absorbent articles is that the stretch laminate should exhibit a low force relaxation.
Stretch laminates (particularly for use in disposable absorbent articles) ideally should exhibit high breathability with a minimal amount of force relaxation. Force relaxation quantifies an elastomer's loss of force as a result of a constant strain and hold at a predetermined strain, temperature, and time. Many diaper components comprise a stretch laminate. The elastic character of the component often improves the fit and function of the diaper. For example, elasticized side panels are common on disposable absorbent articles such as pant-type diapers. The elasticized side panels provide a snug, conforming fit that translates to improved containment of exudates. During wear, the elasticized side panels are maintained in an elongated state, and the elasticized side panel may exert an unload force. Over time, the unload force may diminish. If the elasticized side panel loses too much force, the diaper is prone to sagging which can result in increased leakage.
Another factor to be considered in stretch laminate construction is the hysteresis properties of the elastomeric material. The elastomeric material should not exhibit unnecessarily large hysteresis area under the curve since this evidences the presence of lower unload forces at lower strains (e.g., lower unload forces near strain exhibited in actual product use and application). Lower unload forces could indirectly effect the product fit. One approach to determine the improved properties of stretch laminate is by calculating the load (i.e., 200% strain) to unload (i.e., 50% return load strain). It is desirable to provide a stretch laminate with a low load to unload ratio.
Accordingly, it would be desirable to provide a disposable absorbent article comprising a stretch laminate that exhibits a requisite degree of breathability while maintaining a low degree of force relaxation. The breathable stretch laminate may exhibit a high degree of extensibility with a minimal degree of set. It would also be desirable to provide a method for making such a disposable absorbent article. It is also desirable to provide a disposable absorbent article comprising a breathable stretch laminate that exhibits a requisite degree of breathability and requiring low load forces while maintaining a low degree of force relaxation.